The commercial switched-mode power supply industry was beginning to grow during the 1970s, and the theory and technology of switched-mode conversion was being understood as part of the academic discipline of Power Electronics. The Power Electronics Group of the California Institute of Technology (Caltech) in California, USA developed the models for the three basic DC-to-DC switching regulator topologies already developed, namely the buck, boost and buck-boost converters. From this work stemmed the modeling and analysis method called state-space averaging which allowed the theoretical prediction of a converters frequency response, and therefore a better understanding of a switched-mode regulator's feedback loop and stability criteria. Further work at Caltech, especially by Slobodan Ćuk (1976) produced a fourth member of the basic DC-to-DC switching regulators which has been described as an optimum topology because of its symmetrical structure and non-pulsating input and output currents. This topology DC-to-DC switching regulator is now commonly known as the Ćuk converter.
The theorem known as the maximum power theorem (Jacobi's Law) states: “Maximum power is transferred when the internal resistance of the source equals the resistance of the load, when the external resistance can be varied, and the internal resistance is constant.”
Solar power is a clean and renewable source of energy. Solar power, or solar energy, is the technology of obtaining usable energy from the light of the sun. A photovoltaic cell is a device for converting light energy into electricity. Photovoltaic cells are often used specifically to receive sunlight (and are called solar cells), but may respond to light from other sources. A solar cell array or module or panel is a group of solar cells electrically connected and packaged together. While the interest in solar power is high, the high cost of producing solar cells and arrays coupled with the traditionally low energy efficiency of these devices prevents widespread usage of solar power. Given the variations in sunlight (clouds, rain, sunrise, sunset, altitude, latitude, etc.), solar power is an example of an unstable energy source. Unstable power sources may include natural power sources, but may also include man-made power sources. As with solar energy, minimizing power loss is a significant challenge when attempting to extract and/or convert energy from unstable power sources to stable, useable forms of power. Other examples of power sources include, but are not limited to, wind, water, heat, tidal forces, heat (e.g., thermal couple), hydrogen power generation, gas power generation, radioactive, mechanical deformation, piezo-electric, and motion (e.g., human motion such as walking, running, etc.). Other power sources may be stable (providing an essentially constant power but variable in magnitude).
Prior techniques have been employed to improve the efficiency of solar cells. One of the earliest improvements was the addition of a battery to a solar cell circuit to load level the electrical output from the circuit during times of increased or decreased solar intensity. In itself, a photovoltaic or solar array can supply electrical power directly to an electrical load. However, a major drawback of such a configuration is the diurnal variance of the solar intensity. For instance, during daylight operation, a solar cell produces excess power while during nighttime or periods of reduced sunlight there is little or no power supplied from the solar cell. In the simplest electrical load leveling scenario, the battery is charged by the solar cell during periods of high solar radiation, e.g., daylight, and the energy stored in the battery is then used to supply electrical power during nighttime periods.
A single solar cell normally produces a voltage and current much less than the typical requirement of an electrical load. For instance, a typical conventional solar cell provides between 0.2 and 1.4 Volts of electrical potential and 0.1 to 5.0 Amperes of current, depending on the type of solar cell and the ambient conditions under which it is operating, e.g., direct sunlight cloudy/rainy conditions, etc. An electrical load typically requires anywhere between 5-48 Volts (V) and 0.1-20 Amperes (A). The industry's standard method of overcoming this mismatch of electrical source to load is to arrange a number of solar cells in series to provide the needed voltage requirement and arrange in parallel to provide the needed current requirement. These arrangements are susceptible if the output of individual cells within the solar cell array is not identical. These differences have a negative effect on the array's ability to efficiently convert solar energy into electrical energy. The array's output voltage or current will drop and the array may not function to specification. For example, a common practice it is to configure a solar cell array for an output voltage of 17 V to provide the necessary 12 V to a battery. The additional 5 V provides a safety margin for the variation in solar cell manufacturing and/or solar cell operation (e.g., reduced sunlight conditions, temperature variations within the array, or just dirty cells within the array).
Continuing with this scenario, assuming that the current produced by traditional solar cell arrays is constant, the solar cell array loses efficiency due to the panel array being 5 volts higher than the battery voltage. For example, a solar cell array rated for 75 Watts (W) at 17 Volts will have maximum current of 75 divided by 17, which equals 4.41 Amperes. During direct sunlight, the solar cell array may actually produce 17V and 4.41 A. However, given that the battery is rated at 12V, in this scenario, the power transferred will only be 12V at 4.41 A, which equals 52.94 W and results in a power loss of about 30%. This margin creates a significant power loss; and is typical of what is seen in actual installations where the cell array is connected directly to the batteries, however, it is not desirable to reduce the margin voltage provided by the solar cell array because under reduced sunlight conditions, the voltage potential produced by the solar cell array will drop due to low electron generation, and thus might not be able to charge the battery or will consume power from the battery that it was intended to charge.
FIG. 1 illustrates a prior art system for generating solar power. Photovoltaic (PV) cells 12, 14, and 16 are connected in series to a load 26 through protective circuitry 22 (such as a diode). In the example discussed above, a protection circuit that would prevent the reverse flow of power into the array (e.g., protective circuitry 22) could convert a 17V, 4.41 A input from a solar cell array (e.g., PV cells 12-16) to a 12V and 4.41 A output in order to charge a 12 V battery, which is a significant amount of power loss.
Recent developments in switching converter technology include a technique referred to as maximum power point tracking (MPPT), discussed in U.S. Pat. No. 6,844,739 to Kasai et al.